See cancer for the biology of the disease, as well as a list of malignant diseases.

Oncology is the branch of medicine that studies tumors (cancer) and seeks to understand their development, diagnosis, treatment, and prevention. A physician who practices oncology is an oncologist. The term originates from the Greek onkos (ογκος), meaning bulk, mass, or tumor and the suffix -ology, meaning "study of."

Blood tests, including Tumor markers, which can increase the suspicion of certain types of tumors or even be pathognomonic of a particular disease.

Apart from in diagnosis, these modalities (especially imaging by CT scanning) are often used to determine operability, i.e. whether it is surgically possible to remove a tumor in its entirety.

Generally, a "tissue diagnosis" (from a biopsy) is considered essential for the proper identification of cancer. When this is not possible, empirical therapy (without an exact diagnosis) may be given, based on the available evidence (e.g. history, x-rays and scans.)

Occasionally, a metastatic lump or pathological lymph node is found (typically in the neck) for which a primary tumor cannot be found. This situation is referred to as " carcinoma of unknown primary", and again, treatment is empirical, based on past experience of the most likely origin.

It depends completely on the nature of the tumor identified what kind of therapeutical intervention will be necessary. Certain disorders will require immediate admission and chemotherapy (such as ALL or AML), while others will be followed up with regular physical examination and blood tests.

Often, surgery is attempted to remove a tumor entirely. This is only feasible when there is some degree of certainty that the tumor can in fact be removed. When it is certain that parts will remain, curative surgery is often impossible, e.g. when there are metastases elsewhere, or when the tumor has invaded a structure that cannot be operated upon without risking the patient's life. Occasionally surgery can improve survival even if not all tumour tissue has been removed; the procedure is referred to as "debulking" (i.e. reducing the overall amount of tumour tissue). Surgery is also used for the palliative treatment of some of cancers, e.g. to relieve biliary obstruction, or to relieve the problems associated with some cerebral tumours. The risks of surgery must be weighed up against the benefits.

Chemotherapy and radiotherapy are used as a first-line radical therapy in a number of malignancies. They are also used for adjuvant therapy, i.e. when the macroscopic tumor has already been completely removed surgically but there is a reasonable statistical risk that it will recur. Chemotherapy and radiotherapy are commonly used for palliation, where disease is clearly incurable: in this situation the aim is to improve the quality of and prolong life.

Hormone manipulation is well established, particularly in the treatment of breast and prostate cancer.

A large segment of the oncologist's workload is the following-up of cancer patients who have been successfully treated. For some cancers, early identification of recurrence, with prompt treatment, can lead to better survival and quality of life. It depends on the nature of the cancer whether the follow-up lasts a number of years or remains "life long".

Although 50% of all cancer cases diagnosed achieve curation, a large number of cancer patients will die from the disease. There may be ongoing issues with symptom control associated with progressive cancer. These problems may include pain, nausea, anorexia, fatigue, immobility, and depression. Not all issues are strictly physical: personal dignity may be affected. Moral and
spiritual issues are also important.

While many of these problems fall within the remit of the oncologist, palliative care has matured into a separate, closely allied speciality to address the problems associated with advanced disease. Palliative care is an essential part of the multidisciplinary cancer care team. Palliative care services may be less hospital-based than oncology, with nurses and doctors who are able to visit the patient at home.

These issues are closely related to the patients' personality, religion, culture, personal, and family life. The answers are rarely black and white. It requires a degree of sensitivity and very good communication on the part of the oncology team to address these problems properly.

There is a tremendous amount of research being conducted on all frontiers of oncology, ranging from cancer cell biology to chemotherapy treatment regimens and optimal palliative care and pain relief. This makes oncology an exciting and continuously changing field.

Many cancer patients seek extra help from complementary and alternative therapies, which fall outside of conventional medicine. Most complementary therapies do not have a firm scientific or evidence base. Some patients undoubtedly find complementary therapies helpful while they are undergoing conventional treatment.

While most complementary therapies are probably harmless, they can be expensive. They may also be positively harmful if the patient forgoes conventional treatment altogether, in order to follow alternative regimens. Some alternative regimens are undoubtedly hazardous.

N.B. In the UK, the majority of oncologists are known as Clinical Oncologists, and are fully qualified to practice both chemotherapy and radiotherapy. In most other countries these disciplines are more clearly segregated.

Oncologists may additionally subspecialise in one or two tumour types e.g: radiation oncologist with an interest in Head and Neck, and Breast cancer.